to be honest buy what you like the look of as eldar can rock in all areas but suffer from old codex fever. So you can get good if your practise and think but its all about your style.

I was very much counter attacks so I had falcons full of fire dragons, shining spears, autarch on bike, dire avengers with the occasional wave serpent up until this year. Now I play necrons and again Im doing it my way...

I'd say, get Autarch AND Farseer. They both have very low base costs so it should be easy to fit two in a force. Farseers are some of the best Psykers in the game, though they do do suffer from Codex creep (lack of that is) as does the rest of the Eldar book.

Dark Reapers are nasty against Space Marines due to their ability to negate 3+ Power Armour.Eldar Rangers are the best snipers in the game and get even better if upgraded to Pathfinders (2+ cover saves, rending on 5s AND 6s. HIGH Ballistic Skill).Howling Banshees are fast thanks to fleet and hit hard because they ALL have power weapons (and they have so many attacks that their weak strength of 3 usually isn't a problem).

It depends on your regular opponents really. If Space Marines, then Dark Reapers, Pathfinders, Howling Banshees and a Farseer with Doom and Guide to give you rerolls works well.

in regards to my regular players I play against imperial guard and tau. the tau player has lots... and lots... of crisis suits...and a few rail guns. as far as I know only fire dragons are any good against those sort of things

I would suggest taking a Farseer, at least 2 Squads of Dire Avengers, 0-1 Guardians, 1-2 Elites, 1-2 Heavy Support.I've used something like this to good effect:

HQ:Farseer, Guide, Doom, Runes of Warding, Spirit Stones - 130He will join the large Dire Avenger, Guiding them and Dooming whoever the Howling Banshees are in combat with, or the Dire Avengers.Elites:6 Striking Scorpions, Exarch, Biting Blade, Stalker, Shadowstrike - 138These guys will infiltrate or outflank, as you see fit - good for anti horde/weaker armoured vehicles.6 Howling Banshees, Exarch, Executioner, War Shout, Acrobatic - 128These are your main anti-marine unit. Doom the enemy they're fighting, as this will help rack up the kills meaning less will be killed in return.Troops:5 Dire Avengers - 60This unit will sit on your home objective - could be replaced with Guardians, to provide anti-tank.5 Dire Avengers - 60This unit can be used to delay enemy units, pick off smaller units, or help another squad.10 Dire Avengers, Exarch, Power Weapon, Shimmershield, Defend, Bladestorm - 177These will be used in support of the Banshees, shooting at a different target before charging them.Heavy Support:1 War Walker, Eldar Missile Launcher, Bright Lance, Spirit Stones - 85This unit will either outflank, to shoot Tank's rear armour, or move into terrain.

This puts you at 778 Points, leaving you room for a Wave Serpent or two, or some more anti-tank

The Tau will never be a good matchup for you as they can outrange you and your units are too frail to wither the amount of fire they bring. But to answer your question, get a Wave Serpent and a HQ next. I would suggest a farseer with either doom, fortune and runes of warding.